1. Technical Field
The present invention relates to a method and apparatus for a hydraulic system for aircraft, and more particularly to such a system having improved backup or reserve capability.
2. Background Art
In modern day aircraft, a hydraulic system is commonly used to provide power for various operating components of the aircraft, such as wing flaps, control surfaces, thrust reversers, and also steering and brakes for the landing gear. For reasons of safety, it is quite common to provide redundant hydraulic systems. Thus, if one of the hydraulic systems is damaged or malfunctions, another hydraulic system could take over to provide adequate power. For components such as control surfaces, there may be as many as three or more separate hydraulic systems to provide a yet greater margin of safety, but for other functions such multiple redundancy may not be required.
Another technique in providing a backup system is to supply a single hydraulic system itself with a reserve supply, so that even though a portion of a single hydraulic system is damaged, the reserve supply may be used at least to a limited extent. More specifically, in many aircraft, there is a reservoir of hydraulic fluid that has a standpipe which has an inlet opening that is positioned a moderate distance above the floor of the reservoir and closer to a central location in the reservoir. Thus, if the aircraft experiences negative G loads, so that the fluid in the reservoir tends to rise toward the top of the reservoir, the inlet of the standpipe will still be surrounded by hydraulic fluid, thus avoiding the potential problem of the hydraulic pumps ingesting air from the interior of the reservoir.
While the primary purpose of such a reservoir arrangement is to avoid drawing air into the hydraulic system under negative G loading, this particular reservoir arrangement has been used in the prior art as part of a backup subsystem for that particular hydraulic system. In normal operation, one or more pumps will draw fluid from the reservoir and direct this fluid to a certain use location (i.e. to operate the thrust reversers, flaps, etc.). This same fluid will be recirculated back to the reservoir, with the volume of fluid in the reservoir normally remaining constant, and under normal G loading, will be well above the level of the standpipe.
However, let it be assumed that there is a leak in one of the lines of the hydraulic system. As the pump continues to deliver fluid, the level of the reservoir will drop. The aircraft's monitoring system will normally note the loss of fluid, and as this comes to the pilot's attention, he may shut off that particular hydraulic system in the hopes of avoiding further loss of fluid.
However, let it be assumed that fluid continues to be lost until the level in the reservoir reaches the height of the inlet to the standpipe. At this point, there will be no further fluid flow from the reservoir, so the fluid that remains below the level of the standpipe inlet would not yet be lost. In some prior art aircraft, this fluid that remains in the reservoir below the level of the standpipe can be made available by utilizing an alternate reservoir outlet opening which is located at the bottom wall of the reservoir. In addition, there is provided a valve isolation system which would limit the flow of this backup supply of fluid to a particular isolated system of the aircraft for which this backup system is intended.
While there are advantages to such a system, there is a continuing need in aircraft design to provide such systems in a manner to provide an overall balance of desirable characteristics, such as effectiveness and reliability of operation, simplicity of design, keeping the overall weight of the system within reasonable limits, compatibility with other systems of the aircraft, and also convenience and ease of pilot operation. Therefore, it is an object of the present invention to provide such a hydraulic system with a desirable balance of the features noted above.
A general search of the patent literature has disclosed a number of fluid systems for aircraft and other devices. While some of these are believed to have at best marginal relevance to the present invention, these are being noted to insure that there is a full disclosure of all possibly relevant prior art brought to the attention of the applicant. These are as follows. U.S. Pat. No. 1,390,446, Jerdone Jr., discloses a fuel reserve supply system where there is a main fuel tank and a reserve fuel tank. There is a system by which the fuel can be moved from one tank to another in a predetermined manner.
U.S. Pat. No. 2,678,004, Harris, shows a control means for liquid fuel pumps. More particularly, there is an isolating valve which becomes operative when there is a fracture of an associated pipe.
U.S. Pat. No. 2,866,315, Schakel, shows a hydraulic system for a variable exhaust nozzle for a jet engine. The hydraulic system moves the vanes of the nozzle inwardly or outwardly to vary the area of the nozzle so as to optimize the thrust developed. However, in the event that there is a loss of pressure in the hydraulic system, the force of the gaseous exhaust would tend to move the vanes to a more open position, which would normally not be the optimum position for efficient operation. The hydraulic system is arranged so that if there is a loss of pressure, the vanes are automatically locked in the more constricted operating condition. Upon restoration of pressure to the hydraulic system, the locking mechanism is decommissioned so that the vanes can again be moved inwardly or outwardly.
U.S. Pat. No. 2,834,182, Culbertson, shows a control system for the exhaust nozzle of a jet engine. This patent relates essentially to an improved emergency control apparatus comprising an emergency electrical power circuit.
U.S. Pat. No. 2,836,230, Cruckshank, discloses an emergency fuel pumping system, where the discharges of the main and reheat fuel boost pumps are interconnected with a pressure responsive valve means, for selectively controlling the discharge from the reheat fuel boost pump.
U.S. Pat. No. 2,871,656, Johnson, discloses a thrust reverser where the apparatus is controlled by a temperature sensitive control mechanism.
U.S. Pat. No. 2,932,164, Watson, shows a thrust reverser system that is "fail-safe", in that it will retract due to the forces applied to the blades of the thrust reverser by the jet stream if the reverser actuating mechanism fails.
U.S. Pat. No. 2,940,516, Muraszew, discloses an emergency fuel system where the emergency control unit is brought into operation to bypass the normal fuel controls and to provide an adjustable throttle to control the engine during an emergency.
U.S. Pat. No. 2,994,193, Friedmann, shows a thrust reverser for jet engines that has a fail-safe feature. If the hydraulic power system of the aircraft fails, the rearward flow of the exhaust gases will automatically return the blockage flaps to the normal position to allow unopposed flow of the exhaust gases outwardly of the exhaust nozzle.
U.S. Pat. No. 2,996,881, Gardiner, shows a thrust reverser for a jet engine having a fail-safe feature in that if the controls should fail, the thrust reverser will automatically be positioned to provide full forward thrust.
U.S. Pat. No. 3,050,937, James et al, shows another thrust reverser for a jet engine, where in the event that there is a power failure, the thrust reverser elements are driven toward their retracted or forward-thrust position by the force of the discharging gases.
U.S. Pat. No. 3,230,706, Tripp, is directed to the problem of leakage in the propellant system of a space vehicle. In the event that there is a large leak, all of the injector line valves are closed and then selectively opened, so that the major offender may be detected.
U.S. Pat. No. 3,273,116, Quiros et al, illustrates a safety system for hydraulic brakes in automobile vehicles. When a leak is detected, it is isolated from other portions of the system so that the intact portions of the system can remain operative.
U.S. Pat. No. 3,722,217, Reynolds et al, discloses an emergency hydraulic power supply where there is a hot gas turbine driven from a fuel supply to supply the hydraulic power.
U.S. Pat. No. 3,939,383, Alm, discloses a liquid level monitoring device to detect leakage from a body of liquid.
U.S. Pat. No. 4,033,115, Baits, discloses an emergency hydraulic system where there is a hot gas turbine to drive a hydraulic pump.